[eagle] Re: 70 cm Receiver IF Spurs - Problem Solved

[Juan Rivera]

Hello Lyle, Jim, and folks,

The uglier this gets the more interesting I find it!  I hope no one is
getting disheartened.  After all, this is why we all got involved in the
first place, isn't it?  If it was easy it would be boring.

I just cooked up an idea for a noisy power source that could be used during
payload development and test.  I stuck my write up and a block diagram at
the end of my current log.  

Here's the link:
http://www.juanr.com/pages/hobbies/ham_radio/Eagle/Spur_Suppression.htm

What do you think?

Shielding the CAN-Do inductor obviously won't solve all these problems but
it might buy back 2 inches of space and that's worth the effort.

Lyle,

My suspicion is that the two VCOs on this receiver are also the most
susceptible to radiated EMI from CAN-Do.  I can't say for sure but it looks
that way to me.  Both are in metal cans soldered to ground.  The closest one
was about 2 inches away from the CAN-Do inductor.  I've more than doubled
that distance to 4-1/4 inches to get away from it.  That's a lot of EMI!

73,

Juan
WA6HTP


-----Original Message-----
From: Lyle Johnson [mailto:kk7p at wavecable.com] 
Sent: Sunday, June 17, 2007 7:44 PM
To: juan-rivera at sbcglobal.net
Cc: Chuck Green; David Smith; Dave Black (Work); Dave Black (Home);
eagle at amsat.org; Samsonoff at Mac. Com; Juan.Rivera (Work)
Subject: Re: [eagle] Re: 70 cm Receiver IF Spurs - Problem Solved

Hello Juan!

I have been watching all of this pretty much in the background for a 
variety of reasons. IN addition to the CAN-Do! noise that you've noted, 
we are also faced with a few power system unknowns that are likely to 
dwarf the CAN-Do! conducted noise:

1) The spacecraft power will probably be coming from a distributed set 
of sources  that will be switching themselves on and off bus, with 
hysteresis in their switching logic.

2) The various power sources may not be synchronized in terms of their 
switching frequencies.

3) As a result, well have both switching regulator noise from the 
regulators themselves, as well as varying source impedance as the 
various power sources attach and remove themselves from the power bus 
(due to spacecraft spin, varying power demand from transmitters, ...).

4) The power system is not designed, so it is very hard to characterize it.

 From all of this, I think that all modules need to have excellent 
on-board regulation and assume dirty power in terms of both switching 
noise and source impedance.  I don't have numbers for this, but perhaps 
a source impedance of 0.05 to 1 ohm is not unrealistic.  Similarly, 
switching noise amplitudes on the order of 1 volt are likely.

If we can shield the CAN-Do! inductor to reduce its radiation, we 
should. This will be a retrofit, of course.  The dynamic magnetic 
environment is not known at this time, and I think we've been assuming 
that most radiated energy would be from the antennas, along with 
"leakage" from the power supply switching system.

***

In a previous life, I had to deal with systems that included radios with 
VCOs that were susceptible to external fields. In the end, we had to the 
VCOs had to be shielded to make them essentially immune to such fields.

Since we don;t yet know what the power system will be like in terms of 
conducted or radiated energy, we have to assume a pretty dismal picture 
for both of these components and try to design our modules to be 
insensitive to them to the extent practical.

Lou, any ideas on the power system and how it might look/behave/targets 
for noise, load regulation, etc?  Any measurements available on the 
cap/cell charge/discharge distributed power system modules?

73,

Lyle KK7P